Burning MORE cals running slower?

OK, this seemed very strange to me. I tried something new--running soooooooo slowly that I kept my heart rate under 130 (I'm 49 years old, so my max is about 176 and my aerobic threshold is 105). In 60 minutes (running like this for about 30 minutes and walking/stretching the other 30), I burned 770 cals, according to my heart rate monitor (average HR 122). Previously, I had been averaging about 10 cals/minute in some pretty intense exercise (trail running with steep hills, playing soccer) so I'd burn about 600 in an hour (average HR anywhere from 136 to 142)

Nothing else has changed that I can think of, so I'm really puzzled as to why a so much less intense workout would would give me a higher burn. Any suggestions will be appreciated.

TIA.

Oh, and FYI: it is really "hard" to run so slowly because I want to speed up! It is a very cool experience, however, if you can take the strange looks you get from people who are obviously wondering why you are running slower than you can walk!

I thought about it more and I think my memory is wrong--I burned 696 running/hiking almost 60 minutes, and that 770 came when my workout was 67 minutes long. . . probably just lousy record keeping, b/c it sure does not make sense otherwise!